Inkjet printing mechanisms use cartridges, often called "pens," which shoot drops of liquid colorant, referred to generally herein as "ink," onto a page. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. To print an image, the printhead is propelled back and forth across the page, shooting drops of ink in a desired pattern as it moves. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text).
To clean and protect the printhead, typically a "service station" mechanism is mounted within the printer chassis so the printhead can be moved over the station for maintenance. For storage, or during non-printing periods, the service stations usually include a capping system which hermetically seals the printhead nozzles from contaminants and drying. Some caps are also designed to facilitate priming by being connected to a pumping unit that draws a vacuum on the printhead. During operation, clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a process known as "spitting," with the waste ink being collected in a "spittoon" reservoir portion of the service station. After spitting, uncapping, or occasionally during printing, most service stations have an elastomeric wiper that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the printhead.
To print an image, the printhead is scanned back and forth across a printzone above the sheet, with the pen shooting drops of ink as it moves. By selectively energizing the resistors as the printhead moves across the sheet, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text). The nozzles are typically arranged in linear arrays usually located side-by-side on the printhead, parallel to one another, and perpendicular to the scanning direction, with the length of the nozzle arrays defining a print swath or band. That is, if all the nozzles of one array were continually fired as the printhead made one complete traverse through the printzone, a band or swath of ink would appear on the sheet. The width of this band is known as the "swath width" of the pen, the maximum pattern of ink which can be laid down in a single pass. The media is moved through the printzone, typically one swath width at a time, although some print schemes move the media incrementally by for instance, halves or quarters of a swath width for each printhead pass to obtain a shingled drop placement which enhances the appearance of the final image.
Inkjet printers designed for the home market often have a variety of conflicting design criteria. For example, the home market dictates that an inkjet printer be designed for high volume manufacture and delivery at the lowest possible cost, with better than average print quality along with maximized ease of use. With continuing increases in printer performance, the challenge of maintaining a balance between these conflicting design criteria also increases. For example, printer performance has progressed to the point where designs are being considered that use four separate monochromatic printheads, resulting in a total of over 1200 nozzles that produce ink drops so small that they approximate a mist.
Such high resolution printing requires very tight manufacturing tolerances on these new pens; however, maintaining such tight tolerances is often difficult when also trying to achieve a satisfactory manufacturing yield of the new pens. Indeed, the attributes which enhance pen performance dictate even tighter process controls, which unfortunately result in a lower pen yield as pens are scrapped out because they do not meet these high quality standards. To compensate for high scrap-out rates, the cost of the pens which are ultimately sold is increased. Thus, it would be desirable to find a way to economically control pens with slight deviations without sacrificing print quality, resulting in higher pen yields (a lower scrap-out rate) and lower prices for consumers.
Moreover, the multiple number of pens in these new printer designs, as well as the microscopic size of their ink droplets, has made it unreasonable to expect consumers to perform any type of pen alignment procedure. In the past, earlier printers having larger drop volumes printed a test pattern for the consumer to review and then select the optimal pen alignment pattern. Unfortunately, the individual small droplets of the new pens are difficult to see, and the fine pitch of the printhead nozzles, that is, the greater number of dots per inch ("dpi" rating) laid down during printing, further increases the difficulty of this task. From this predicament, where advances in print quality have rendered consumer pen alignment to be a nearly impossible task, evolved the concept of closed-loop inkjet printing.
In closed loop inkjet printing, sensors are used to determine a particular attribute of interest, with the printer then using the sensor signal as an input to adjust the particular attribute. For pen alignment, a sensor may be used to measure the position of ink drops produced from each printhead. The printer then uses this information to adjust the timing of energizing the firing resistors to bring the resulting droplets into alignment. In such a closed loop system, user intervention is no longer required, so ease of use is maximized.
Closed loop inkjet printing may also increase pen yield, by allowing the printer to compensate for deviations between individual pens, which otherwise would have been scrapped out as failing to meet tight quality control standards. Drop volume is a good example of this type of trade-off. In the past, to maintain hue control the specifications for drop volume had relatively tight tolerances. In a closed loop system, the actual color balance may be monitored and then compensated with the printer firing control system. Thus, the design tolerances on the drop volume may be loosened, allowing more pens to pass through quality control which increases pen yield. A higher pen yield benefits consumers by allowing manufacturers to produce higher volumes, which results in lower pen costs for consumers.
In the past, closed loop inkjet printing systems have been too costly for the home printer market, although they have proved feasible on higher end products. For example, in the DesignJet.RTM. 755 inkjet plotter, and the HP Color Copier 210 machine, both produced by the Hewlett-Packard Company of Palo Alto, Calif., the pens have been aligned using an optical sensor. The DesignJet.RTM. 755 plotter used an optical sensor which may be purchased from the Hewlett-Packard Company of Palo Alto, Calif., as part no. C3195-60002, referred to herein as the "HP '002" sensor. The HP Color Copier 210 machine uses an optical sensor which may be purchased from the Hewlett-Packard Company as part no. C5302-60014, referred to herein as the "HP '014" sensor. The HP '014 sensor is similar in function to the HP '002 sensor, but the HP '014 sensor uses an additional green light emitting diode (LED) and a more product-specific packaging to better fit the design of the HP Color Copier 210 machine. Both of these higher end machines have relatively low production volumes, but their higher market costs justify the addition of these relatively expensive sensors.
FIG. 19 is a schematic diagram illustrating the optical construction of the HP '002 sensor, with the HP '014 sensor differing from the HP '002 sensor primarily in signal processing. The HP '014 sensor uses two green LEDs to boost the signal level, so no additional external amplification is needed. Additionally, a variable DC (direct current) offset is incorporated into the HP '014 system to compensate for signal drift. The HP '002 sensor has a blue LED B which generates a blue light B1, and a green LED G which generates a green light G1, whereas the HP '014 sensor (not shown) uses two green LEDs. The blue light stream B1 and the green light stream G1 impact along location D on print media M, and then reflect off the media M as light rays B2 and G2 through a lens L, which focuses this light as rays B3 and G3 for receipt by a photodiode P.
Upon receiving the focused light B3 and G3, the photodiode P generates a sensor signal S which is supplied to the printer controller C. In response to the photodiode sensor signal S, and positional data S1 received from an encoder E on the printhead carriage or on the media advance roller (not shown), the printer controller C adjusts a firing signal F sent to the printhead resistors adjacent nozzles N, to adjust the ink droplet output. Due to the spectral reflectance of the colored inks, the blue LED B is used to detect the presence of yellow ink on the media M, whereas the green LED G is used to detect the presence of cyan and magenta ink, with either diode being used to detect black ink. Thus, the printer controller C, given the input signal S from the photodiode P, in combination with encoder position signal S1 from the encoder E, can determine whether a dot or group of dots landed at a desired location in a test pattern printed on the media M.
Historically, blue LEDs have been weak illuminators. Indeed, the designers of the DesignJet.RTM. 755 plotter went to great lengths in signal processing strategies to compensate for this frail blue illumination. The HP Color Copier 210 machine designers faced the same problem and decided to forego directly sensing yellow ink, instead using two green LEDs with color mixing for yellow detection. While brighter blue LEDs have been available in the past, they were prohibitively expensive, even for use in the lower volume, high-end products. For example, the blue LED used in the HP '002 sensor had an intensity of 15 mcd ("milli-candles"). To increase the sensor signal from this dim blue light source, a 100.times. amplifier was required to boost this signal by 100 times. However, since the amplifier was external to the photodiode portion of the HP '002 sensor, this amplifier configuration was susceptible to propagated noise. Moreover, the offset imposed by this 100.times. amplifier further complicated the signal processing by requiring that the signal be AC (alternating current) coupled. Additionally, a 10-bit A/D (analog-to-digital) signal converter was needed to obtain adequate resolution with this still relatively low signal.
The HP '014 sensor used in the HP Color Copier 210 machine includes the same optics as the HP '002 sensor used in the DesignJet.RTM. 755 plotter, however, the HP '014 sensor is more compact, tailored for ease in assembly, and is roughly 40% the size of the HP '002 sensor. Both the HP '002 and '014 sensors are non-pulsed DC (direct current) sensors, that is, the LEDs are turned on and remain on through the entire scan of the sensor across the media. Signal samples are spatially triggered by the state changes of the encoder strip, which provides feedback to the printer controller about the carriage position across the scan. At the relatively low carriage speed used for the optical scanning, the time required to sample the data is small compared to the total time between each encoder state change. To prevent overheating the LEDs during a scan, the DC forward current through the LED is limited. Since illumination increases with increasing forward current, this current limitation to prevent overheating constrains the brightness of the LED to a value less than the maximum possible.
The HP '014 sensor designers avoided the blue LED problem by using a new way to detect yellow ink with green LEDs. Specifically, yellow ink was detected by placing drops of magenta ink on top of a yellow ink bar when performing a pen alignment routine. The magenta ink migrates through yellow ink to the edges of the yellow bar to change spectral reflectance of the yellow bar so the edges of the bar can be detected when illuminated by the green LEDs. Unfortunately, this yellow ink detection scheme has results which are media dependent. That is, the mixing of the two inks (magenta and yellow) is greatly influenced by the surface properties of media. For use in the home printer market, the media may range from a special photo quality glossy paper, down to a brown lunch sack, fabric, or anything in between. While minimum ink migration may occur on the glossy, photo-type media, a high degree of migration will occur through the paper sack or fabric. Thus, ink mixing to determine drop placement becomes quite risky in the home market, because these earlier printers had no way of knowing which type of media had been used during the pen alignment routine.
To address this media identification problem, a media detect sensor was placed adjacent to the media path through the printer, such as on the media pick pivoting mechanism or on the media input tray. The media detect sensor reads an invisible-ink code pre-printed on the media. This code enables the printer to compensate for the orientation, size and type of media by adjusting print modes for optimum print quality to compensate for these variances in the media supply, without requiring any customer intervention. Both the drop detect and media detect sensors use a light-to-voltage (LVC) converter and one or more light emitting diodes (LED), with each sensor being dependent on a housing to orient the optical elements and shield the LVC from ambient light. In an effort to provide consumers with economical inkjet printing mechanisms that produce high quality images, the costs associated with implementing both sensors were analyzed. Surprisingly, a substantial portion of the cost of both sensors is not related to the sensing unit itself, but instead, is a function of the costs associated with interconnecting the sensors to the printer controller and keeping a greater number of distinct parts in inventory.
Actually, media type detection is not present in the majority of inkjet printers on the commercial market today. Most printers use an open-loop process, relying on an operator to select the type of media through the software driver of their computer. Thus there is no assurance that the media actually in the input tray corresponds to the type selected for a particular print request, and unfortunately, printing with an incorrectly selected media often produces poor quality images. Compounding this problem is the fact that most users never change the media type settings at all, and most are not even aware that these settings even exist. Therefore, the typical user always prints with a default setting of the plain paper-normal mode. This is unfortunate because if a user inserts expensive photo media into the printer, the resulting images are substandard when the normal mode rather than a photo mode is selected, leaving the user effectively wasting the expensive photo media. Besides photo media, transparencies also yield particularly poor image quality when they are printed on in the plain paper-normal mode.
The problem of distinguishing transparencies from paper was addressed in the Hewlett-Packard Company's DeskJet 2000C Professional Series Color Inkjet Printer, which uses an infrared reflective sensor to determine the presence of transparencies. This system uses the fact the light passes through the transparencies to distinguish them from photo media and plain paper. While this identification system is simple and relatively low cost, it offers limited identification of the varying types of media available to users.
One proposed system offered what was thought to be an ultimate solution to media type identification. In this system an invisible ink code was printed on each sheet of the media in a location where it was read by a sensor onboard the printer. This code supplied the printer driver with a wealth of information concerning the media type, manufacturer, orientation and properties. The sensor was low in cost, and the system was very reliable in that it totally unburdened the user from media selection through the driver, and insured that the loaded media was correctly identified. Unfortunately, these pre-printed invisible ink codes became visible when they were printed over. The code was then placed in the media margins to avoid this problem, but market demand is pushing inkjet printers into becoming photo generators. Thus, the margins became undesirable artifacts for photographs with a full bleed printing, that is, being printed to the edge of the paper. Thus, even placing the code in what used to have been a margin when printed over in full-bleed printing mode created a severe print defect.
Another sensor system for media type determination used a combination transmissive/reflective sensor. The reflective portion of the sensor had two receptors at differing angles with respect to the surface of the media. By looking at the transmissive detector, a transparency could be detected due to the passage of light through the transparency. The two reflective sensors were used to measure the specular reflectance of the media and the diffuse reflectance of the media, respectively. By analyzing the ratio of these two reflectance values, specific media types were identified. To implement this system, a database was required comprising a look-up table of the reflective ratios which were correlated with the various types of media. Unfortunately, new, non-characterized media was often misidentified, leading to print quality degradation. Finally, one of the worst shortcomings of this system was that several different types of media could generate the same reflectance ratio, yet have totally different print mode classifications.
Thus, it would be desirable to provide a multi-function optical sensing system for determining information about ink droplets printed on media, and for determining information about the type of media, so the printing mechanism can adjust future printing for optimal images without requiring user intervention.